1. Field of the Invention
This invention relates to a composite papermaking fabric which includes an upper fabric and a lower fabric which are superimposed and connected to one another by binding threads.
2. State of the Prior Art
Composite papermaking fabrics are used on the wet end of papermaking machines for dewatering the fiber suspension which has been deposited on the upper fabric as a slurry as the composite papermaking fabric is introduced into the entrance of the machine. The paper web which is thus formed from the fibers and filling materials of the slurry or pulp is transported through the machine on the composite papermaking fabric which is moved as an endless belt to the press on the papermaking machine.
The composite papermaking fabric not only has to fulfill the basic requirements of dewatering and forming the paper web at the desired rate, but the structure of the fabric must impart to the paper web the desired characteristics of the surface of the paper such as its smoothness and ability to be printed. This in turn is governed by the marking of the paper surface which occurs by the contact with the threads of the upper fabric under the pressure caused by belt or fabric tension which can be fairly great, exceeding 200 Newtons per centimeter. These high tensions also result in high frictional forces causing rapid wear of the lower or running surface of the composite fabric.
Therefore, the advantages and disadvantages of using a one layer fabric versus using a double layer fabric are well known, and even making the choice between a single fabric and a composite fabric, a balance must be struck between producing a paper having an excellent surface quality and maintaining adequate strength in the fabric for long wearing qualities. The finer surface quality of the paper demands fine, thin threads, while wearing and fabric stability require larger diameter wires and a different weave pattern.
While it is apparent that the composite fabric allows the use of a finer wire size and weave pattern for the upper fabric upon which the paper web is formed and the use of coarser wire and fabric for the lower running surface of the composite papermaking fabric, years of development have gone into various ways of interconnecting the upper and lower fabrics by binding warp threads or binding weft threads.
Composite fabrics are already known in which the two fabric layers are interconnected by fabric-born threads of the upper fabric. That is, the interconnecting binding threads are an integral part of the upper fabric. This is shown in European Patent 69 101 and its corresponding U.S. Pat. No. 4,501,303. While this is a significant improvement over the use of independent binding threads, there are fundamental drawbacks with this design. Although the fabric-born binding threads in the upper fabric produce a suitable pattern, they are not simultaneously suited for the lower fabric, thus disturbing the structure of the composite fabric resulting in marking of the paper. Moreover, the binding threads are quickly worn and destroyed because of the relative movement of the two fabrics during its use on the papermaking machine causing friction which in turn results in separation of the two fabric layers from one another. When thicker and more stable binding threads are used to stop the relative movement between the fabric layers, this automatically results in attenuating the marking problem because the thicker the binding threads are, the more the structure of the wire is disturbed.